We have followed the development of the MLR stimulating and antigen presenting cells in mice from the newborn period until the onset of adult immune reactivity. We find a diminished capacity to present antigens and stimulate in the allogeneic MLR, which increases over the first three weeks following birth approaching adult levels at three to four weeks. Cells with classical dendritic lymphoid morphology are absent or present in very small numbers until two weeks of life, when they increase to approach 50% of adult levels by three weeks. Using cloned antigen specific T cells, two day neonates do present antigen and investigations are underway to determine the characteristics of the antigen presenting cell. Evidence has been accumulated that alphafetoprotein, with Cu++ bound to histidine residue #3 is immunosuppressive and its mechanisms of action probably involve inhibition of antigen presenting function.